Supply selection circuit with programmable hysteresis

Abstract

An apparatus ( 205 ) and system ( 200 ) for selecting between power supplies in a redundant system which can be integrated in silicon in which transistors ( 320, 321 ) are used to provide a conduction path between the power supplies and the load, and in which a comparator (305) is used to compare the voltage magnitudes of the power supplies for indicating the largest magnitude and activating the appropriate transistor ( 320, 321 ). Trip points occur when one magnitude becomes larger than the other magnitude by values determined by a programmable hysteresis of the comparator ( 305 ). The hysteresis is programmable via an external programming device which can include resistive elements (R 1 , R 2 , R 3 ) coupled in a voltage divider arrangement. Each of the transistor switches ( 320, 321 ) can include a pair of series coupled transistor switches for use with larger hysteresis requirements.

What is claimed:  
     
         1 . A device for selectively coupling power supplies with a load in a plurality supply system having a first power supply and a second power supply, said device comprising: 
 a selector circuit comprising: 
 a first input for receiving from said first power supply a signal indicative of a voltage magnitude of said first power supply;  
 a second input for receiving from said second power supply a signal indicative of a voltage magnitude of said second power supply; and  
 a comparator having a programmable hysteresis and coupled to said inputs and responsive to said voltage magnitude signals indicating a first control signal for determining that said first voltage magnitude is larger than said second voltage magnitude and responsive to said voltage magnitude signals indicating a second control signal for determining that said second voltage magnitude is larger than said second voltage magnitude corresponding to said hysteresis;  
   a first switch responsive to said first control signal for coupling said first power supply with said load; and    a second switch responsive to said second control signal for coupling said second power supply with said load.    
     
     
         2 . The device of  claim 1 , wherein said first switch is a transistor having a drain connected to said first power supply, a source coupled with said load, and a gate coupled with an output of said selector circuit for receiving said first control signal and responsive thereto for enabling a conduction path between said first power supply and said load via said drain and said source, and said second switch is a transistor having a drain connected to said second power supply, a source coupled with said load, and a gate coupled with a further output of said selector circuit for receiving said second control signal and responsive thereto for enabling a conduction path between said second power supply and said load via said second transistor drain and source.  
     
     
         3 . The device of  claim 1 , wherein said first switch includes a first pair of transistors coupled in inverse series between said first power supply and said load such that a gate of each of said first transistor pair is coupled with an output of said selector circuit for receiving said first control signal and responsive thereto for enabling a conduction path between said first power supply and said load, and said second switch including a second pair of transistors coupled in inverse series between said second power supply and said load such that a gate of each of said second transistor pair is coupled with a further output of said selector circuit for receiving said second control signal and responsive thereto for enabling a conduction path between said second power supply and said load.  
     
     
         4 . The device of  claim 1 , wherein said switches are operable for turn-off more quickly than for turn-on.  
     
     
         5 . The device of  claim 1  further including a transistor coupled in series between said switches and the load and operable for soft turn-on control and fault disconnection such that when said transistor is one of OFF and partially ON the body diode is non-conductive.  
     
     
         6 . The device of  claim 1 , wherein said first switch and said second switch are metal oxide semiconductor field effect transistors (MOSFET).  
     
     
         7 . The device of  claim 6 , wherein said first MOSFET has an integral diode which is forwardly biased from said first power supply to said load, and said second MOSFET has an integral diode which is forwardly biased from said second power supply to said load.  
     
     
         8 . The device of  claim 7 , wherein said hysteresis is less than the voltage drop of said integral diodes.  
     
     
         9 . The device of  claim 1 , wherein said selector circuit is integrated in a silicon chip, said first and second inputs comprise first and second external I/O pins of said integrated circuit.  
     
     
         10 . The device of  claim 9 , wherein the substrate of said integrated circuit is connectable with said first power supply via a third I/O pin responsive to said first control signal from said first switch and with said second power supply via said third I/O pin responsive to said second control signal from said second switch.  
     
     
         11 . The device of  claim 1  further including a programming unit connected between said selector circuit and said power supplies for programming said hysteresis.  
     
     
         12 . The device of  claim 11  wherein said programming unit comprises: 
 a first resistor connected between said selector circuit first input and said first power supply;  
 a second resistor connected between said selector circuit second input and said second power supply; and  
 a third resistor connected between said first input and said second input, said resistors having selectable resistance cooperable for programming said hysteresis.  
 
     
     
         13 . An apparatus for selectively coupling one of a plurality of power supplies with a load via a power switch, said apparatus comprising: 
 a plurality of inputs for receiving respective signals indicative of a voltage magnitude of a corresponding power supply;    a comparator having a programmable hysteresis and coupled to said inputs and responsive to said voltage magnitude signals indicating respective control signals for determining that one voltage magnitude is larger than other voltage magnitudes; and    an output providing said control signals to said power switch for enabling coupling of said load with a respective power supply responsive to a corresponding control signal indicating a larger magnitude.    
     
     
         14 . The apparatus of  claim 13  in a silicon chip, each of said inputs comprise an external I/O pin of said integrated circuit.  
     
     
         15 . The apparatus of  claim 14 , wherein the substrate of said integrated chip is connectable with a respective power supply responsive to a corresponding control signal via said power switch.  
     
     
         16 . The apparatus of  claim 13  further including a programming unit comprising a voltage divider connected to said inputs between said comparator and said power supplies for programming said hysteresis.  
     
     
         17 . The apparatus of  claim 16  wherein said voltage divider comprises: 
 a resistive element connectable between respective inputs and power supplies; and  
 a further resistive element connectable between said inputs, said resistive elements having selectable resistance cooperable for programming said hysteresis.  
 
     
     
         18 . The apparatus of  claim 13 , wherein said hysteresis is less than a body diode of a metal oxide semi conductor field effect transistor.  
     
     
         19 . A method for selecting between power supplies in a system having a plurality of power supplies for driving a load, comprising: 
 using respective transistors to provide a conduction path between each of said power supplies and said load;    comparing a voltage magnitude of each of said power supplies for indicating the largest magnitude;    activating a corresponding transistor for an indication that said power supply magnitude is larger than other power supply magnitudes, wherein activating trip points occur when one magnitude becomes larger than the other magnitudes by values determined by a programmable hysteresis.    
     
     
         20 . The method of  claim 19  further including programming said hysteresis to be less than a voltage drop of the integral diode of said transistors.  
     
     
         21 . The method of  claim 19 , wherein said transistors are metal oxide semiconductor field effect transistors.  
     
     
         22 . The method of  claim 19 , wherein said comparing and said activating are implemented in a integrated chip, and said programming is implemented in a voltage divider circuit coupled between said power supplies and said integrated chip.